1. Field of the Invention
The invention pertains to the field of the industrial electrolytic production of aluminum. More particularly, the invention pertains to the automated control by process variables in the Hall-Héroult method of primary aluminum production.
2. Description of Related Art
The production of primary aluminum metal is a highly energy-intensive industry. A substantial portion of the cost of aluminum production is in the enormous amount of electrical energy required. Increasing energy costs and increasing requirements for low levels of polluting emissions place increasing demands on the primary aluminum production industry. There is therefore always a need for methods that improve the energy efficiency of the aluminum production process and decrease fluoride emissions, including greenhouse perfluorocarbons (PFCs). The Hall-Héroult process for primary aluminum production, which is used by all major industrial aluminum producers, utilizes direct electrical current passing through a molten chemically-modified cryolite electrolyte, or “bath”, to produce aluminum metal from alumina (Al2O3). In the process, the alumina is dissolved in an electrolyte composed primarily of molten cryolite (Na3AlF6) and other additives such as excess aluminum fluoride (AlF3) and calcium fluoride (CaF2) at temperatures above 900° C. As current is passed through the electrolyte, aluminum metal is deposited at the molten aluminum cathode, and oxygen evolves at the surface of a solid carbon anode and combines with the carbon to produce mostly carbon dioxide and lesser amounts of carbon monoxide gas. The dissolved alumina in the electrolytic cells, or “pots”, is depleted in direct proportion to the amount of aluminum metal produced.
The concentration of alumina in the electrolytic bath is of critical importance to the efficiency of the aluminum production process. As the alumina concentration in the electrolytic bath decreases, a point is reached where a phenomenon known as an “anode effect” occurs, typically in the concentration range of 1.5% to 2% alumina. When an anode effect occurs, the voltage drop across the cell, which is normally between about 4 to 4.5 volts, may rise very rapidly to a level of about 15-30 volts. The actual concentration of alumina in the electrolyte at the onset of this effect depends upon the critical anode current density. Other variables, such as temperature and composition of the electrolyte, also play a role, but anode effects usually occur at an alumina concentration below about 2% by weight of the electrolyte. A cell in the anode effect state becomes less productive and consumes a large amount of power, thus seriously compromising the efficiency of the process. Additionally, during anode effects, the cryolite electrolyte enters into chemical reactions at the anode leading to the production of gaseous fluorinated products including perfluorocarbons (PFCs) and hydrogen fluoride (HF). The emission of PFCs has become a point of concern since PFCs have thousands of times more infrared radiative capture capacity than CO2. Hydrogen fluoride (HF) releases to the environment are especially deleterious to plant life. The release of greenhouse PFCs and HF to the atmosphere is becoming increasingly restricted by environmental legislation.
Although the anode effect presents serious problems in the control of aluminum reduction cells, this phenomenon is generally a less serious commercial problem than the overfeeding of alumina to the cell. A cell with a continuing excess of added alumina may enter an operational stage commonly termed a “sick pot” or “sick cell.” The upper practical limit for alumina concentration for operation is approximately 4%, above which alumina no longer dissolves sufficiently fast. The ideal operational concentration and solubility of alumina in the bath therefore falls within a narrow window of about 2-4% alumina. If a cell is overfed, all of the alumina does not immediately dissolve in the electrolyte and a fraction of it therefore tends to settle at the top of the metal cathode or at the bottom of the cell, thereby seriously increasing a cell's electrical resistance and promoting non-uniform current distribution. These effects also decrease a cell's cathode life. Un-dissolved alumina that settles to the bottom of a cell is called cathode “sludge” or “muck” and is difficult to remove quickly by the dissolution process. Additionally, residual un-dissolved alumina muck on the bottom carbon cathode surface promotes erosive effects, since the alumina itself is extremely abrasive and scours the carbon cathode surface due to the motion of alumina particles by the magneto-hydrodynamics of the metal pad. Hence cathode life is significantly reduced. This necessitates capital expenditures for rebuilding the cathode shell after it fails. During a failure episode, iron levels may rise either gradually or sharply, thereby degrading the quality of the aluminum produced during the remaining shortened life of the cathode. While it may take a short period, on the order of one to ten minutes, to extinguish an anode effect, an overfed cell takes considerably longer time to allow carbon cathode muck levels to be decreased. Thus it is has been usual industrial practice to operate as much as practical nearer the lower part of the operational alumina concentration range to specifically avoid the problems of cathode sludge or muck. Indeed, many automated control strategies have in the past attempted to promote at least one anode effect about every day or so, specifically to prevent the overfeeding of alumina ore to the cell. However, in light of the increased sensitivity to the atmospheric release of un-capturable perfluorocarbons (PFCs), which accompanies anode effects, as well as the metal production decrease, this strategy needed to be replaced to conform with more stringent environmental laws and to address the increased cost of electrical power.
Ideally, to maintain optimal production efficiency, the concentration of dissolved alumina is preferably held at moderate levels as much as possible by adding alumina at the same rate it is being consumed in a cell. Unfortunately this is not always possible to achieve due to the physical characteristics of the pots and the difficulty of accurately monitoring the actual in situ concentration of dissolved alumina in the electrolyte bath on a continuous real time basis. In practice, the voltage drop across each cell and the current passing through the potline are used to compute a pseudo-resistance (PR) variable to estimate the state of the cells and the need for the addition of alumina and changes in pot voltage. Cell voltage changes are achieved by controlling the distance between the carbon anode surface and the aluminum cathode surface (anode/cathode gap). The cells in a commercial aluminum production plant are connected in electrical DC series and most often number a hundred or more (referred to collectively as a potline). The measured raw data that is sampled by a computer or microprocessor to assess the status of the individual pots is typically limited to the voltage drops (V) across the individual cells and the simultaneous amperage through the potline (A). Extrapolating from these measured parameters to calculate the dissolved alumina level on a real time basis is a goal of great practical interest and is a complex problem that has been the subject of much research. Each cell behaves differently at a given moment as a result of differences in numerous factors including the bath alumina level and its rate of change, the age of the cell, the electrolyte composition, the anode-cathode distance, the condition of the anodes and cathodes, and the operating temperature. Thus the relationship between changes in the current-voltage profile and the dissolved alumina level may be somewhat different for each cell in a potline. This situation is further complicated by a number of factors which affect the operating current and voltage of the cells. As a result of the interdependence of a large number of cells in a potline, the potline amperage generally fluctuates to a greater or lesser degree because of changes occurring in one or more cells at any given instant, including occasional power changes in the rectifier. Cells in a potline typically experience voltage changes from two primary sources: the internal changing levels of alumina (assuming other bath variables do not change significantly in a short time period) and the external fluctuating potline amperages. Cell voltage changes over several seconds or more are affected mostly by fluctuating potline amperages and not the subtle voltages from very small changes in bath alumina levels, when these levels are not so low that an anode effect is pending within a minute or so. Amperage levels may also fluctuate whenever power loads in the rectifier change.
In present practice, a cell's voltage/amperage data is sampled over time and processed to yield a variable known as pseudo-resistance (PR) that attempts to factor out voltage changes from external fluctuating potline amperages, while retaining the changes indicative of the cell's alumina level (see e.g. Dirth et al., U.S. Pat. No. 3,573,179).
The definition of a cell's pseudo-resistance value is:PR=(V−I)/A  (1)                where                    V=cell voltage at a given instant            A=line amperage at the same instant            I=the extrapolated cell voltage at zero amps (an estimated and generally inaccurate value is arbitrarily chosen)                        
Processes for automated alumina feed, using feedback from measured pot voltage and line amperage parameters from the electrolytic cells, have been described in the following patents: French patent FR 1 457 746, in which the variation of the internal pseudo-resistance of the cell is used as the key parameter reflecting the concentration of alumina, and French patent FR 1 506 463, in which control is based on measurement of the time elapsing between halting the alumina supply and the appearance of an anode effect. More recently, a process based on controlling the alumina content has been described in particular by Wakaizumi et al. (U.S. Pat. No. 4,126,525).
In addition to attempted efforts to achieve accurate control of the alumina concentration in the cells, it is common practice to automate the control of the anode-cathode distance or gap to optimize cell voltage levels. Adjustments are typically made by raising or lowering the carbon anodes in the bath. The anode-cathode distance has a strong effect on what is commonly labeled as pot noise, so this variable is also tied to the pseudo-resistance (PR) process control variable already discussed. If the anode-cathode gap has been either squeezed too much or driven to higher than optimal levels, a PR sourced calculated noise level is used to make a statistically derived anode/cathode adjustment to the cell to maintain thermal balance in order to reduce the loss in production efficiencies that can occur if high temperature excursions are encountered.
A Hall-Héroult electrolytic cell is not a classic resistor. Hence the relationship between cell voltage and cell current is not, strictly speaking, linear over the entire amperage range with a zero/zero intercept. In an operating potline, the amperage fluctuates to some degree about an average operating level which is never anywhere near an amperage of zero. In potline practice the relationship between voltage and amperage is most often a linear one for all practical purposes. A cell's total impedance does include classic ohmic resistances including the electrical connectors, anode and cathode bus conductors, carbon anode drop, cathode drop, and the cell's intrinsic electrolytic resistivity, anode gas bubble resistance, and an ohmic component of the electrolytic bath itself. The voltage drop across the anode-cathode gap, where there is a small separation between the two electrodes, includes components that are not ohmic in nature. They generally include cathode-anode over-voltages (dependent upon alumina level, bath ratio, and temperature) and a back electromotive force (emf) value which is not the same as the extrapolated intercept value (I) of the operating voltage/amperage linear relationship. Direct measurement of the back emf necessitates lowering the amperage to almost zero values, which is neither practical nor useful for an operating potline, especially since the voltage/amperage is not linear over the entire range starting at zero amps. The alumina concentration over-voltages at operating potline amperages may vary rapidly in a relatively short time period, since alumina concentration changes quickly if alumina consumption is not compensated by the correct and commensurate amount of alumina feed. However, the rate of increase of the over-voltage component due to decreasing changes in the alumina concentration may be on the order of magnitude of a few millivolts per minute, which is very daunting to nearly impossible for the PR variable to confidently predict in the short time period of several minutes. This required sensitivity is simply not within the grasp of the PR variable. There is therefore an ongoing and pressing need for an accurate in situ method which is capable of accurately estimating in a short time period the amount of alumina dissolved in the electrolyte of a given cell and which is also relatively impervious to the noise and complications created by interference from changes in other process variables.
A plot of a cell's voltage (V) versus potline amperage (A) over a short time period is mostly linear with a positive non-zero intercept (I) value that is almost impossible to accurately measure in a practical way at any given moment. The slope of this line is another way to describe the PR variable. It seems that the choice of the PR variable as the control variable was the logical one, when automated control was first instituted because the linear relationship of V and A was so obvious to everyone. Thus it may have happened that the choice for using another control variable less subject to intrinsic error was overlooked, since the slope (i.e. PR) relationship between voltage and amperage was so obvious. There is a general agreement that the use of the term pseudo-resistance is an appropriate one for the slope of this relationship. PR is not a true resistance value even though it often incorrectly appears in the literature bearing units of micro-ohms. It is well known that different combinations of bath variables and anode/cathode gaps produce different linear relationships of voltage versus amperage. This has been shown to be true from many years of experience using the PR variable in potline control since the advent of process computer control decades ago. It became generally obvious from the beginning of the automated control period that there would be utility in calculating a slope value (PR) for a given voltage/amperage data point to obtain a hopefully useful predictor of the state of a cell, as potline amperages varied for reasons pointed out previously. It seemed reasonable and prudent to select the obvious pseudo-resistance (PR) value as a control variable in potline automated control at that time, since the linear voltage relationship was so well known in all quarters. The PR variable was adopted throughout the industry as a first step in describing the state of the cell as voltage/amperage data is sampled from cells in a potline. The PR variable is still very widely, if not exclusively, used for process control in modern aluminum smelters.
Downstream processing of the PR variable to obtain a better image of the state of a pot and what changes may be taking place on a real time basis is common practice and takes many forms. These innovations have taken on sophisticated roles and have been used for improving aluminum process control over the years and have resulted in significant cell voltage reductions and alumina feed control improvements. The industry has made large strides in reducing unit energy consumption as well as environmental fluoride emissions using these methods, but there is a never ending need in the art for further progress.